REPORT IN BRIEF
Space Studies Board
September 2015
DIVISION ON ENGINEERING AND PHYSICAL SCIENCES
Review of the MEPAG Report on Mars Special
Regions
P
lanetary protection refers to the practice of both protecting solar system bodies such as planets, moons, and asteroids from contamination by terrestrial life and protecting Earth from possibly harmful life
forms that may be returned from other solar system bodies. Planetary protection policies are not static but evolve over time based on the increasing
knowledge and understanding of both planetary environments and the
physical and chemical limits of terrestrial life. The Committee on Space
Research (COSPAR) of the International Council for Science formulates
and maintains international consensus planetary protection policies using technical recommendations from its member scientific organizations,
including the National Academies of Sciences, Engineering, and Medicine
in the United States.
One aspect of these international planetary protection policies is that specific constraints are
placed on the development and operation of spacecraft with the potential to enter so-called
Special Regions on Mars where conditions conducive to microbial growth might exist. At
NASA’s request, the community-based Mars Exploration Program Analysis Group (MEPAG)
established the Special Regions Science Analysis Group (SR-SAG2) in October 2013 to re-examine the quantitative definition of a Special Region and propose modifications based upon
the latest scientific results. Immediately before the formal publication of SR-SAG2’s report in
the November 2014 issue of the journal Astrobiology, NASA’s Science Mission Directorate asked
the Academies’ Space Studies Board (SSB) to review the SR-SAG2 report. At about the same
time, the European Space Agency (ESA) issued a parallel request to the European Science
Foundation (ESF). To avoid duplication of effort, the SSB and ESF established a joint committee to review the SR-SAG2 report’s conclusions and develop recommendations for how the
planetary protection requirements for Mars Special Regions should be updated. The resulting
report, which will provide input to COSPAR as it revises and updates its planetary protection
policies, agrees with many of SR-SAG2’s findings including retaining the currently specified
environmental parameter limits for life. However, there are some cases where more detailed
consideration is warranted. The report provides a list of suggestions for future research and
puts forward an expanded definition of Special Regions. This new definition treats geological
features that have a probable but unquantified association with liquid water, including potential sources of methane, as Uncertain Regions, to be treated as Special Regions until proven
otherwise.
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IDENTIFYING MARS SPECIAL REGIONS
Mars Special Regions refer to places where conditions might
be warm and wet enough to support microbes carried by a
spacecraft from Earth or where there is a high potential for the
existence of extant martian life. Current COSPAR planetary
protection policy sets the parameters for Special Regions to
include areas that maintain temperatures greater than -25°C
and a water activity (the fraction of usable water content in a
system) between 0.5 and 1.0 over a 500 year timescale. The
review committee agrees that these environmental parameters are still appropriate; however, the identification of these
regions remains problematic. First, there is a lack of detailed
knowledge of the physical and chemical conditions of the surface and subsurface of Mars at various scales, particularly the
microscale. Second, current understanding of the ability of
life to propagate is limited. It is not known if one, ten, or a million cells from a single species are required for propagation
in an extraterrestrial environment. Alternatively, propagation may only be possible for microbial communities that are
collections of many different species. As current and future
space missions send back new information about the martian
environment and new techniques in biology are developed
to address these questions, the current practice of reassessing
the concept of a Special Region every 2 years is both appropriate and essential.
POTENTIAL FOR TERRESTRIAL LIFEFORMS TO
SURVIVE ON MARS
While SR-SAG2’s assessment of the potential for terrestrial life
to survive and proliferate on Mars is comprehensive, there are
several remaining issues that merit further study in order to
better identify Special Regions.
Limits of Terrestrial Life
To date, there have been no experimental attempts to determine whether the number and type of cells that remain on a
spacecraft after sterilization and launch are sufficient to estab-
lish a population of microorganisms within a Mars Special Region. There is a need for scientific investigations that deepen
our knowledge about the limits of life with a focus on survivability, adaptation, and evolution under martian conditions.
Small-Scale Microbial Habitats
The definition of Mars Special Regions is based on temperature
and humidity conditions that are measured on spatial scales of
many tens of kilometers that do not necessarily reflect the conditions within microscale niches (millimeters and smaller) that
can be potential habitats for microbial communities. Microenvironments underneath rocks, for example, could provide
favorable conditions for establishing life on Mars, potentially
leading to the recognition of Special Regions where landscape-scale temperature and humidity conditions do not meet
the standard definition. To identify Special Regions across the
full range of spatial scales relevant to microorganisms, a better
understanding of the temperature and water activity of possible microenvironments on Mars is necessary.
Translocation of Terrestrial Contamination
A potential problem with designating Special Regions on Mars
is that viable microorganisms that survive the trip to Mars
could be transported into distant Special Regions by atmospheric processes, landslides, avalanches, meteorite impact
ejecta, or lander impact ejecta. The issue of translocation is
especially worthy of consideration because if survival is possible during atmospheric transport, the designation of Special
Regions may become difficult or even irrelevant. The viability
of translocating microbes could be confirmed or rejected in
terrestrial Mars simulation chambers where transport processes such as dust storms can be studied.
MARTIAN GEOLOGICAL FEATURES
POTENTIALLY RELATED TO SPECIAL REGIONS
There are several types of geological phenomena observed
on Mars that may indicate the presence of liquid water and
Mid-latitude martian gullies at 37.46˚S, 222.95˚E
exhibiting erosional alcoves, channels, and depositional aprons, all geological features that appear
to be actively evolving and resemble landforms
that on the Earth are formed by water. Observations of gullies over the last decade reveal occasional mass wasting and show that they are currently active. However, present-day activity occurs
when it is too cold for liquid water and is likely
driven by dry granular processes involving CO2
frost. Scale of photo is approximately 1.5 km from
top to bottom. Image credit: NASA/JPL/University
of Arizona.
could be considered as possible Special Regions. For
example, Recurring Slope Lineae (RSLs) are narrow
dark markings on steep, rocky slopes that appear to
lengthen during warm seasons and fade in cold seasons. The growth and retreat of RSLs suggests that
seasonal water seepage may be involved, but this has
not yet been demonstrated. Other notable features include slope streaks, gullies, craters, and caves. While
these specific terrains are identified as Special Regions
in both the COSPAR policy and in the SR-SAG2 report,
they are best regarded as Uncertain Regions. These
Uncertain Regions should be treated as Special Regions
until proven otherwise, with the final classification depending on the latest scientific knowledge about the
environmental parameters of a specific site.
The SR-SAG2 report concludes that the detection of
indigenous organic compounds on Mars at very low
concentrations should not be used to distinguish Special Regions. However, it is appropriate that special
consideration be given to methane, recently detected
in episodic plumes near the surface of Mars. Methane could indicate the presence of living organisms
on Mars, but it could also result from non-biological
processes involving liquid water. In either case, it may
be appropriate to designate the regions where methane is being produced, once identified, as Uncertain
Regions.
HUMAN SPACEFLIGHT
Even though planning for human missions to Mars is
in its infancy, the planetary protection implications of
sending astronauts to Mars raises profound questions
at the intersection of science, engineering, technology,
project management, and public policy. Understanding the implications of humans on Mars and the ability
of human systems to meet COPSPAR requirements is
essential to ensuring that nations can continue to conduct science investigations without worrying about
contaminants introduced by human missions. It is important to make it clear that human missions to Mars
will not be exempt from COSPAR planetary protection
requirements and that these policies may prevent humans from landing in or entering areas that may be
Special Regions.
DISTINGUISHING BETWEEN UNCERTAIN
AND SPECIAL REGIONS
While maps are helpful to illustrate the distribution
of relevant landforms or other surface features, they
only represent a snapshot in time and are of limited
use in identifying Uncertain or Special Regions. The
knowledge presented in maps is often incomplete,
available only at a fixed scale, and subject to change as
Recurring slope lineae (RSL) in a crater on the floor of central Valles Marineris. RSL are narrow, dark markings on steep, rocky slopes in the equatorial
and southern mid-latitude regions of Mars. They appear to incrementally
lengthen during warm seasons and fade in cold seasons, best explained
as a result of seasonal water seepage by terrestrial analogy, although the
origin of water is unknown. Photo is 193 m wide. Image credit: NASA/JPL/
University of Arizona.
new information is obtained. Therefore, maps are most useful if accompanied by cautionary remarks on their limitations. Because both
small-scale microbial habitats and larger geological features could
host conditions favorable for terrestrial life, a multiscale approach
is necessary when classifying Special Regions and choosing landing
sites for future missions. All available data should be utilized when
certifying landing sites on Mars and determining whether Uncertain
or Special Regions exist within the proposed landing ellipse.
Uncertain Regions in planned landing ellipses should be evaluated on
a case-by-case basis as part of the site selection process. The goal of
such an evaluation is to determine whether or not the landing ellipse
contains water, ice, or subsurface discontinuities with a potential to
contain hydrated minerals that could be accessed via a landing malfunction or by the operation of subsurface-penetrating devices (e.g.,
drills). As an example, landing site analysis will likely include a geological analysis, drawing on the Mars geologic literature (covering a
broad range of relevant topics, including ground truth at previous
lander locations) as well as orbital imaging, infrared spectroscopy,
gamma-ray spectroscopy, and ground-penetrating radar sounding of
the specific region.
This diagram illustrates the
known ways that methane
(CH4 ) could be added to or removed from the atmosphere,
processes known respectively
as methane sources and sinks.
NASA’s Curiosity Mars rover is
searching for methane traces
as a potential sign of life (a
biomarker), as well as to gain
an understanding of modern
surface and subsurface organic
processes on Mars. Curiosity
has indeed detected fluctuations in methane concentration
in the atmosphere, suggesting
that both methane sources
and sinks are currently at work
in the martian environment.
Detecting methane does not
necessarily mean the presence
of life. Methane can be generated by microbes as well as
by non-biological processes,
such as geochemical reactions,
sunlight-induced
reactions
(photochemistry), or delayed
release from subsurface methane stores. Reactions between
water and olivine (or pyroxene)
and rock can generate methane. Ultraviolet (UV) radiation can induce reactions in a process called photochemistry to produce methane from
other organic compounds that are themselves formed by biological or non-biological means, such as comet dust falling on Mars. Recent or
ancient subsurface methane may be stored within lattice-structured methane hydrates called clathrates and released over time, a source of
modern atmospheric methane that may have formed in the past. Concentrations of atmospheric methane can drop due to redistribution or
photochemical sinks. Wind on Mars can quickly reduce localize methane concentrations from an individual source. Just as methane can be generated through photochemistry, it can be broken down in the same way, sunlight-induced reactions oxidizing the methane through intermediary
chemicals like formaldehyde and methanol into carbon dioxide, the predominant component of the martian atmosphere. Image credit: NASA/
JPL-Caltech/SAM-GSFC/University of Michigan
COMMITTEE TO REVIEW THE MEPAG REPORT ON MARS SPECIAL REGIONS: Petra Rettberg, German Aerospace Center, Chair; Alexandre
Anesio, University of Bristol; Victor Baker, University of Arizona; John A. Baross, University of Washington, Seattle; Sherry L. Cady, Pacific
Northwest National Laboratory; Christine M. Foreman, Montana State University; Ernst Hauber, German Aerospace Center; Gian Gabriele
Ori, IRSPS; David Pearce, Northumbria University; Nilton Renno, University of Michigan; Gary Ruvkun, Harvard Medical School; Birgit
Sattler, University of Innsbruck; Mark P. Saunders, NASA (retired); Dirk Wagner, German Research Center for Geosciences Helmholtz Centre
Potsdam; Frances Westall, Centre National de la Recherche Scientifique
STAFF: Emmanouil Detsis, Science Officer, European Science Foundation; David H. Smith, Study Director, Space Studies Board; Nicolas
Walter, Senior Science Officer, European Science Foundation; Andrea Rebholz, Program Coordinator, Space Studies Board; Danielle
Youngsmith, Lloyd V. Berkner Space Policy Intern, Space Studies Board; Michael H. Moloney, Director, Space Studies Board; Jean-Claude
Worms, Head Science Support Office, European Science Foundation
This study is based on work supported by a contract between the National Academies of Sciences, Engineering, and Medicine and the
National Aeronautics and Space Administration and work supported by a contract between the European Science Foundation and the
European Space Agency. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s)
and do not necessarily reflect the views of the organizations or agencies that provided support for the project.
Copies of this report are available free of charge from http://www.nap.edu.
Report issued September 2015. Permission granted to reproduce this brief in its entirety with no additions or alterations.
Permission for images/figures must be obtained from their original source.
© 2015 The National Academy of Sciences